1. Field of the Invention
The present invention relates to a ferritic stainless steel with an aluminum-containing surface which is coated with a blade-like aluminum oxide and the process for manufacturing this stainless steel; and, in particular, to an aluminum-containing stainless steel with improved adhesiveness to catalyst carrier which can be utilized in a honeycomb metal structure for a catalytic converter for automobile exhaust gases or in a catalytic device for purifying combustion gases, and the process for manufacturing this stainless steel.
2. Description of the Prior Art
Conventionally, a honeycombed structure made from a stainless steel filament containing aluminum, with alumina whiskers formed on the surface in order to strengthen the adhesiveness to catalyst carrier, is used as a catalytic converter for automobile exhaust gases. Activated alumina particles are applied as a coating to this honeycombed structure as a catalyst carrier, after which a catalytic metal such as Pt, Rh, or the like is caused to adhere to this coating.
A method by which alumina whiskers can be formed on this surface have been disclosed in Japanese Patent Laid-open Nos. 96726/1981, 152965/1981, and 71898/1982. This technology involves a two-stage process whereby a stainless steel filament is heated for about one minute at 875.degree. to 925.degree. C. in a low oxygen atmosphere with an oxygen partial pressure of 0.75 Torr or less, after which it is subjected to long-term oxidation in air in a temperature range of 870.degree. to 930.degree. C. In addition, in Japanese Patent Laid-open No. 299711/1987 a process is disclosed by the inventors of the present invention whereby a ferrite stainless steel is coated with aluminum then heated under vacuum or in an atmosphere of non-oxidizing gases to diffuse the aluminum, after which it is subjected to long-term oxidation in air in a temperature range of 870.degree. to 930.degree. C.
However, the alumina whiskers on the surface of the stainless steel filament obtained by this type of process are formed and grow through a rather uncommon and unique oxidation process. Under the environment in which a catalytic converter for automobile exhaust gases is used in actual practice, this growth alumina whiskers continues, and the rate of growth is greater than the rate of growth of the more commonly known aluminum oxide protective layers. As a result, the adherence of the catalyst to this surface worsens, and, in addition, the high rate of growth of the alumina whiskers gives an adverse effect on its resistance to oxidation.
In order to improve the material with this type of drawback, the inventors of the present invention had previously proposed a method whereby once the alumina whiskers have been formed their further growth is restrained by a suitable heat treatment to a degree encountered in more common type of aluminum oxide protective layers (Japanese Patent Laid-open No. 166958/1988). However, with this method, after the alumina whiskers have once been formed on the surface by a heat treatment, they must be subjected to a second stage heat treatment at a temperature higher than 975.degree. C.